The prior art is replete with various types of machines having rotating components for industrial, domestic, recreational and other purposes. Because of particular physical phenomenons associated with rotating movements, rotating components part of various types of machines are subjected to particular operational parameters that may be potentially damaging especially when the rotating components reach critical angular values. The potential for subjecting rotating components to damaging conditions is sometimes compounded when the rotating components are used for imparting a rotational movement to material contained therein, such as for mixing, grinding or other purposes.
So-called grinding mills constitute a typical example of a machine having a rotating component, namely a rotating drum that may be subjected to potentially damaging conditions upon operational parameters of the machine meeting pre-determined critical parameter conditions while the rotating drum reaches a critical angular value. Such grinding mills are used extensively for reducing lumps or large pieces of various kinds of material to smaller sizes.
Conventional grinding mills commonly include a hollow cylindrical or frusto-conical shell or drum mounted for rotation about its longitudinal axis. The drum is typically rotatably arranged about two trunnions by two head portions positioned at opposite longitudinal ends of the drum.
Typically, each conical head portion includes a plurality of segments bolted together to form a composite structure. Each head portion is also typically provided an inner annular flange and an outer annular flange for securing the head portions respectively to a trunnion and to the drum.
Also, conventional grinding mills are typically provided with a gear wheel forming part of the gear mechanism that drives the grinding mill. The gear wheel commonly includes a plurality of segmental rim portions that are bolted together to form an annular rim. Gear teeth are cut into the rim and shaped for cooperation with one or more pinions. The annular rim is typically displaced radially outward of the drum by a rib. The rib is usually provided with a plurality of apertures through which bolts may pass to fasten the rib to the outer annular flange of the head portion and the flange of the drum.
The gear wheel typically forms part of a large speed-reducing gear system intended to transmit the power from a prime mover to the grinding mill. The prime mover, in turn, typically includes an electrical prime mover such as synchronous electrical motors or the like having enhanced starting torque characteristics. In order to compensate for enhanced starting torque, the gear wheel typically has a relatively large diameter.
Different diameters and lengths of shells or drums have been used heretofore, and they normally vary in proportion to the capacity of the mill. During rotation of the drum about its longitudinal axis, the material to be ground is carried up the side of the drum to subsequently fall to the bottom of the drum. The grinding occurs principally by attrition and impact within the grinding mill charge.
In the case of ore, the normal function of the grinding mill is to reduce the size of the ore to particles within a fine sieve range for flotation. Grinding mills used for grinding ores or the like optionally use grinding mediums such as pebbles, steel balls, ceramic balls, or the like to assist in the comminuting process as the mill is rotated.
In other circumstances, the ore may be self-grinding. The axial ends of the drum may be open, and the material to be comminuted may be continuously fed into the mill at one end with the comminuted product continuously emerging from the other end.
In view of the abrasive character of the material being ground, the wear on the inside of the grinding mill has heretofore been a serious problem. Hence, in order to protect the drum from the grinding action and to thereby lengthen the life of the grinding mill, the drum is typically provided with a metal or rubber lining. For example, grinding mills have been lined with cast or wrought abrasion-resistant ferrous alloy liners and, in some cases, rubber or ceramic liners. Typically, these liners are segmented due to the weight and size considerations.
Liner assemblies hence typically include a plurality of separate lining components that are usually retained tightly against the interior or the mill shell or drum by mechanical fastening components such as bolts. Some ores, such as taconite, are relatively highly abrasive. In order to maintain continuous operation of the grinding mill, it is necessary to provide a liner for the drum that is highly abrasion-resistant. The liner also should be tough enough to withstand the continuous impact of ore fragments.
Liners inevitably become worn and, hence, no longer effective. In such situations, the liners are typically replaced at periodic intervals. Other types of maintenance and repair also periodically require the grinding mill to be run at speeds considerably slower than the normal running speed or even to stop the rotation movement of the drum altogether.
As a result of mill shut-down over a period of time, the charge within the mill may “freeze” into a generally solidified, hardened or rigid lump. Upon the mill being rotated after a mill shut-down there exists the possibility that the solidified lump will be carried up the side of the drum by the rotation of the latter. In such instances, instead of tumbling in a cascading flow upon reaching the position wherein non-solidified charge would cascade, the mass may eventually detach itself from the inner wall of the drum and fall on an impacting location within the drum.
This may prove to be detrimental to various components of the mill including the lining, heads and bearings thereof. Also, since gear wheels are typically constructed with great accuracy, they may also be subjected to deformation by the impact. As can be appreciated, when the lining is affected or when a tooth in a gear wheel is damaged, the liner and the wheel must be replaced. The cost of the occurrence of such events is very burdensome. Not only is the cost of material and repair involved extensive but the high capitalization costs of plants using large autogenous mills may be mobilized by extended non-productive down-time.
A solidified mass falling from the mill inner wall upon rotation of the latter constitutes a typical example of a rotating component that may be subjected to potentially damaging conditions upon the rotating component reaching a critical angular value. Another example of angle-dependent potentially damaging conditions may result from the potential mismatch between actual load and designed torques.
Indeed, as the mill is rotated to the cascade position wherein the charge starts to tumble, the torque required increases quite considerably as the charge is moved away from the gravity-balanced position on a large radius. Once the charge begins to tumble, the required load torque drops. If the developed motor torque matches the load torque plus the friction torque, then the rotation will be smooth and continuous.
It would be desirable to provide an angle-based protection device for protecting rotating component and corresponding supporting component part of machines. More particularly, in some situations the rotating component defines a critical angular value about which an operational parameter of the machine may be used for predicting the occurrence of a potentially damaging condition for the machine. Also, sometimes the potentially damaging condition for the machine is concurrently more susceptible to happen upon the operational parameter meeting predetermined critical parameter conditions while the rotating component reaches a critical angular displacement value. In such situations, it would be desirable to provide an angle-based protection device for reducing the risk of such potentially damaging conditions occurring.
As mentioned previously, it is some times desirable to run the grinding mill at speeds considerably slower than the normal running speed. Typical examples include for the purpose of assuring proper gear, bearing and shaft alignment when a mill is first being installed, also for inspecting and potentially replacing the mill liner when the mill is empty or to start the mill after it has been stopped with a full charge. This slow running is often referred to as “spotting”, “inching”, “barring” or “turning gear”.
Heretofore, inching has been accomplished in several ways. One of the simplest mechanical device used for inching includes a cable sling arrangement attached to an overhead crane. The cable sling arrangement allows for selective mill rotation. However, such a prior art technique is not precise. Also, it requires continuous use of a crane. Furthermore, it is dangerous to personnel who may be installing or re-lining the mill as slings have a known tendency to break.
Another way to provide for inching uses a low frequency power source to provide power to the stator windings of the typically used three-phase synchronous drive motors. The low frequency power source may be a direct current (DC) supply connected to an inching supplied bus for the motors through a series of electromechanical or static switches to produce stepped low frequency three-phase voltages. These switches are typically referred to as sequencing or commutating switches. The switches, however, are relatively costly.
Inching has heretofore also been accomplished through the use of clutches, the clutches may be partially engaged to cause rotation of the mill at lower speeds. This partial clutch closure for long periods however generates considerable heat in the clutches and requires that the wet clutches be installed and provision made to dissipate the heat generated. Also, typically, an installation using wet clutches is more expensive than one using dry clutches.
Yet, another way to provide for inching is to use a removable hydraulic motor that is placed to engage main mill pinion gear. The present invention is particularly well suited for use with such inching devices. However, it can be appreciated by those skilled in the art that the present invention has broader applications and be used in conjunction with other types of machinery for obtaining an angle-based protection device.